There are different evolutions of akuma (level 0, 1, 2, 3, 4). The akuma don't just evolve randomly, they have to kill a lot of humans before they can evolve.

As they evolve, akuma become more powerful, and the soul held within the akuma becomes more disfigured. Of course, the akuma itself looks different as they evolve.

So the when they say "the meaning behind the akumas' evolution", they're talking about that (see above). Or, at least, that's what I assume.

i don't think so that they have to kill alot of humans...
I think tat like innocence and exorcist, they evolve maybe gotta do something with emotion and the drive to do...(like allen when his innocence evolved)
Somehow I have this theory that Innocence and Noah are related...

"A Level 1 Akuma will advance to a Level 2 once it has fulfilled a substantial quota of kills of humans and exorcists." "After murdering enough people, it (a level 2) can evolve to Level 3." - Taken from wikipedia

Plus, level 1 akuma don't have any emotions, although emotions and individual thought are developed after evolving.

Also, noah and innocence are opposites. Innocence is like poison to the noah, where as the noah are able to destroy innocence. (kinda like a light-shadow relationship)

i can't wait for the next chapter tho i wonder when it'll be all over..........i think there is still alot of chapters less. plus i usuallly think that authors that write really mangas aren't really that great but d. grayma. manga changed that. it was at a really fast pace i liked it